Field of the Disclosure
This disclosure relates to a laser and specifically to a solid state or fiber laser that generates radiation near 183 nm and is suitable for use in inspection of photomasks, reticles, and/or wafers. The laser is preferably a pulsed laser such as a Q-switched laser or a mode-locked laser. This disclosure further relates to an inspection system using a laser operating at a wavelength near 183 nm.
Related Art
Excimer lasers for generating light at 193 nm are well known in the art. Unfortunately, such lasers are not well suited to inspection applications because of their low laser pulse repetition rates and their use of toxic and corrosive gases in their lasing medium, which leads to high cost of ownership.
Solid state and fiber lasers for generating light near 193 nm are also known. Exemplary lasers use two different fundamental wavelengths (e.g. US 2014/0111799 by Lei et al.) or the eighth harmonic of the fundamental (e.g. U.S. Pat. No. 7,623,557 to Tokuhisa et al.), either of which requires lasers or materials that are expensive or are not in high volume production. Another approach (U.S. Pat. No. 5,742,626 to Mead et al.) has not resulted in a commercial product with stable output and high power as required for semiconductor inspection applications (approximately 1 W or more is typically required in a laser that can run continuously for three or more months between service events). Moreover, most of these lasers have very low power output and are limited to laser pulse repetition rates of a few MHz or less.
As semiconductor devices dimensions shrink, the size of the largest particle or pattern defect that can cause a device to fail also shrinks. Hence a need arises for detecting smaller particles and defects on patterned and unpatterned semiconductor wafers. The intensity of light scattered by particles smaller than the wavelength of that light generally scales as a high power of the dimensions of that particle (for example, the total scattered intensity of light from an isolated small spherical particle scales proportional to the sixth power of the diameter of the sphere and inversely proportional to the fourth power of the wavelength). Because of the increased intensity of the scattered light, shorter wavelengths will generally provide better sensitivity for detecting small particles and defects than longer wavelengths.
Since the intensity of light scattered from small particles and defects is generally very low, high illumination intensity is required to produce a signal that can be detected in a very short time. Average light source power levels of 1 W or more may be required. At these high average power levels, a high pulse repetition rate is desirable as the higher the repetition rate, the lower the energy per pulse and hence the lower the risk of damage to the system optics or the article being inspected. High repetition rates are also desirable in high-speed inspection as a high repetition rate (such as about 50 MHz or higher) allows many pulses to be collected for each image resulting in less sensitivity to pulse-to-pulse variations in intensity.
Therefore, a need arises for a laser and preferably to a solid state or fiber laser that generates radiation shorter than 193 nm and is suitable for use in inspection of photomasks, reticles, and/or wafers. Notably, such inspections at high speeds often require minimum laser pulse repetition rates of multiple MHz (e.g. greater than 50 MHz in some cases).